1. Field of the Invention
The present invention relates to a synchronization method for communication systems, and more especially, to a symbol time synchronization method for OFDM systems.
2. Background of the Related Art
Orthogonal frequency division multiplexing (OFDM) is a promising technology for broadband transmission due to its high spectrum efficiency, and its robustness to the effects of multipath fading channels. However, it is sensitive to synchronization errors. As a result, one has to achieve as good synchronization as possible in OFDM transmissions.
Like other communication systems, there are many synchronization issues should be taken into considerations in OFDM systems. First of all, unknown signal delays will introduce symbol time offset (STO) and require coarse symbol time (CST) and fine symbol time (FST) synchronizations. There also exists carrier frequency offset between a transmitter and a receiver so that fractional carrier frequency offset (FCFO), integral carrier frequency offset (ICFO) and residual carrier frequency offset (RCFO) have to be eliminated. In addition, the sampling clocks mismatch between DAC and ADC will introduce sampling clock frequency offset (SCFO).
In J. J. van de Beek, M. Sandell and P. O. Borjesson's “ML estimation of time and frequency offset in OFDM systems,” (IEEE Trans. Signal Process., vol. 45, no. 7, pp. 1800-1805, July 1997), STO and FCFO are jointly estimated by a delayed-correlation algorithm. It is an ML estimation and only good for AWGN channels.
In T. M. Schmidl and D. C. Cox's “Robust frequency and timing synchronization for OFDM,” (IEEE Trans. Commun., vol. 45, no. 12, pp. 1613-1621, December 1997), a new method making use of training symbols in time-domain was proposed. However, its correlation results exhibit uncertain plateau in multipath fading channels.
Some techniques, for example in H. Minn, V. K. Bhargava and K. B. Letaief's “A robust timing and frequency synchronization for OFDM systems,” (IEEE Trans. Wireless Commun., vol. 2, no. 4, pp. 822-839, July 2003), produce good ST (symbol time) performances. However, extra time-domain training symbols are needed.
The techniques mentioned above are applied to AWGN and/or static multiple channel condition, thus will not be suitable for real environments.
Although the technique in K. Ramasubramanian and K. Baum's “An OFDM timing recovery scheme with inherent delay-spread estimation,” (GLOBECOM'01. IEEE. vol. 5, pp. 3111-3115, Nov. 2001.7) can identify ISI-free region in multipath fading channels, for accurate ST estimation, it may involve too many symbols.
In M. Speth, S. Fechtel, G. Fock and H. Meyer's “Optimum receiver design for OFDM-based broadband transmission-part II: a case study,” (IEEE Trans. Commun., vol. 49, no. 4, pp. 571-578, Apr. 2001.8), for FST, channel responses must be estimated first, IFFT is then applied to get the channel impulse responses (CIR) and adjust the symbol boundary. Hence, its computational complexity is high.
The work in D. Lee and K. Cheun's “Coarse symbol synchronization algorithms for OFDM systems in multipath channels,” (IEEE Commun. Letter, vol. 6, no. 10, pp. 446-448, October 2002), treats CST in multipath fading channels.
T. Lv, H. Li and J. Chen's “Joint estimation of symbol timing and carrier frequency offset of OFDM signals over fast time-varying multipath channels,” (IEEE Trans. Signal Process., vol. 53, no. 12, pp. 4526-4535, December 2005) and J. C. Lin, “Maximum-likelihood frame timing instant and frequency offset estimation for OFDM communication over a fast Rayleigh-fading channel,” (IEEE Trans. Vehicular Tech., vol. 52, no. 4, pp. 1049-1062, July 2003) assume that normalized Doppler frequency (NDF) is known. This restricts their applicability.